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Commit | Line | Data |
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1 | ============================================== | |
2 | Ordering I/O writes to memory-mapped addresses | |
3 | ============================================== | |
4 | ||
5 | On some platforms, so-called memory-mapped I/O is weakly ordered. On such | |
6 | platforms, driver writers are responsible for ensuring that I/O writes to | |
7 | memory-mapped addresses on their device arrive in the order intended. This is | |
8 | typically done by reading a 'safe' device or bridge register, causing the I/O | |
9 | chipset to flush pending writes to the device before any reads are posted. A | |
10 | driver would usually use this technique immediately prior to the exit of a | |
11 | critical section of code protected by spinlocks. This would ensure that | |
12 | subsequent writes to I/O space arrived only after all prior writes (much like a | |
13 | memory barrier op, mb(), only with respect to I/O). | |
14 | ||
15 | A more concrete example from a hypothetical device driver:: | |
16 | ||
17 | ... | |
18 | CPU A: spin_lock_irqsave(&dev_lock, flags) | |
19 | CPU A: val = readl(my_status); | |
20 | CPU A: ... | |
21 | CPU A: writel(newval, ring_ptr); | |
22 | CPU A: spin_unlock_irqrestore(&dev_lock, flags) | |
23 | ... | |
24 | CPU B: spin_lock_irqsave(&dev_lock, flags) | |
25 | CPU B: val = readl(my_status); | |
26 | CPU B: ... | |
27 | CPU B: writel(newval2, ring_ptr); | |
28 | CPU B: spin_unlock_irqrestore(&dev_lock, flags) | |
29 | ... | |
30 | ||
31 | In the case above, the device may receive newval2 before it receives newval, | |
32 | which could cause problems. Fixing it is easy enough though:: | |
33 | ||
34 | ... | |
35 | CPU A: spin_lock_irqsave(&dev_lock, flags) | |
36 | CPU A: val = readl(my_status); | |
37 | CPU A: ... | |
38 | CPU A: writel(newval, ring_ptr); | |
39 | CPU A: (void)readl(safe_register); /* maybe a config register? */ | |
40 | CPU A: spin_unlock_irqrestore(&dev_lock, flags) | |
41 | ... | |
42 | CPU B: spin_lock_irqsave(&dev_lock, flags) | |
43 | CPU B: val = readl(my_status); | |
44 | CPU B: ... | |
45 | CPU B: writel(newval2, ring_ptr); | |
46 | CPU B: (void)readl(safe_register); /* maybe a config register? */ | |
47 | CPU B: spin_unlock_irqrestore(&dev_lock, flags) | |
48 | ||
49 | Here, the reads from safe_register will cause the I/O chipset to flush any | |
50 | pending writes before actually posting the read to the chipset, preventing | |
51 | possible data corruption. |